“J hy © Pi be ~~ * VS ee Et ne a ae nd e 
~ , . — ie a te ee, 
> 8 Ya Y, = ¢ i a1 hea Jt) a f 
ie eau ; Fs 
bs 
208 (Spruce and Ph ysiological Botany, 
in which there is no tension, as is the case with tools; it ‘oul See 
simply follow the direction of gravitation, and its apex will _ 
direct itself vertically downwards. But when the growing part eu 
consists of layers of tissue ina state of tension,a change 
takes. place in the tension of the tissues and in the direction  — 
of growth, which causes the under side of the curved part 
to grow more vigorously than the upper side, and conse- ~ 
quently tends to force the apex upwards. The erect growth 
of most stems is a result of this force of geotropism. — 
GENERAL CONDITIONS OF PLANT-LIFE.! 
Very little is at present known with regard to the general 
bearing of plants as respects Aeat; for example, with re- 
gard to their power of conducting heat, or to the changes 
in volume of masses of tissue and individual cells through 
the influence of temperature ; but somewhat more on the 
influence of different degrees of temperature on the various 
vital phenomena of plants. On this point the following 
important laws have been established :—All functions of the 
organs of a plant are brought into play only when the tem- 
yerature rises to a certain height above the freezing-point of 
the sap. When the temperature begins to rise above this 
lower limit, the functions begin to be exercised with an 
increasingly greater energy, until at a certain height the 
point of the greatest vital activity is attained. With a 
further increase of temperature above this point, both the 
rapidity and the energy of the functions again decrease, 
until, at length, they completely cease at a definite maxi- — 
mum of temperature, which can apparently never be per- . 
manently higher than 50° C. ‘ ay 
Beneath a covering of snow, or at a temperature below zero C., oe 
scarcely anything grows. The few perennial plants which blossom with us in 
1 This section is taken in the main from Sachs’s 7extbook of Botany, 
Book ITI., chap. i. . 
